The parasitic inductances of a synchronous rectifier metal oxide semiconductor field effect transistor (MOSFET) cause dissipative ringing and excessive conduction of the integral body diode of the MOSFET, thereby limiting rectification efficiency. In particular, the integral body diode of a MOSFET typically has a slow reverse recovery, e.g., on the order of 100 nanoseconds, resulting in commutation losses which increase as frequency increases. That is, at turn-off of the MOSFET, the energy that was stored in the parasitic inductances of the MOSFET during its conduction period forces the body diode to carry current. During the subsequent body diode reverse recovery period, the parasitic inductances store energy in the opposite polarity. The stored energy is dissipated in a ring between the parasitic capacitances and inductances of the MOSFET. At high frequencies (e.g., above 300 kHz), these commutation losses are so high as to render use of synchronous rectifiers impractical. Hence, Schottky rectifiers, which have significantly higher forward conduction losses, are typically used for high-frequency rectification.
It has been found that one way to avoid body diode conduction during commutation of a synchronous rectifier is to connect a Schottky diode package in parallel, i.e., with like polarity, with the body diode of the synchronous rectifier package, as described in "A MOSFET Resonant Synchronous Rectifier for High-Frequency DC/DC Converters" by W. A. Tabisz, F. C. Lee and D. Y. Chen, 1990 IEEE Power Electronics Specialist Conference Proceedings, Vol. II, pp. 769-779. For low-frequency operation, such a parallel package configuration results in a slightly higher efficiency. However, at high frequencies, parasitic inductances force the body diode to conduct, rather than the parallel Schottky diode, for the whole commutation period, such that rectification efficiency is not improved.
Accordingly, it is desirable to provide a synchronous rectifier package capable of high-efficiency operation over a wide range of frequencies, i.e., for both low- and high-frequency rectification, and thus render high-frequency synchronous rectification practicable. To this end, it is desirable to reduce or eliminate the wire and lead inductances, while minimizing the device resistances.